Griebel signed a lot of his work. He was a Freelancer so self promotion was to his advantage. This especially early on when he first immigrated to the USA.
That promotion wasn't needed after a time as his work became known and sought out for what it was.
The sign of a true artist and craftsman.

His signature was usually a severely plain block letter style. No extra fancy lettering, no Script styles with intertwining loops.
Usually easy to find if it is there but can be very small.

Here are a couple pics
One on a Colt pistol w/a pic of the pistol engraving. The other on a Winchester 1894.

[Linked Image from i.ibb.co]
[Linked Image from i.ibb.co]
[Linked Image from i.ibb.co]